With the overall goal of minimizing the frequency of iatrogenic hypoglycemia - thelimiting factor inthe management of diabetes - this project focuses on themechanisms of hypoglycemia-ossociated autonomicfailure (HAAF) in type 1 diabetes mellitus (T1DM). HAAF posits that episodes of iatrogenic hypoglycemia, by reducing autonomic, including epinephrine, and symptomatic responses to subsequent hypoglycemia, cause the clinical syndrome of hypoglycemia unawareness and, in the setting of absent glucagon responses, that of defective glucose counterregulation and thus a vicious cycle of recurrent hypoglycemia in T1DM. The specific aims are to test four hypotheses in healthy human subjects: 1.Recent antecedent hypoglycemia causes increased blood-to-brain glucose transport ([l-"C]glucose and PET) at a given plasma glucose concentration and thus shifts glycemic thresholds for responses to hypoglycemia to lower plasma glucose concentrations. 2. Any activity, such as exercise, that increases cortisol secretion shifts glycemic thresholds (stepped hypoglycemic clamp technique) for autonomic, including epinephrine, and symptomatic responses to hypoglycemia to lower plasma glucose concentrations, and this is specific for the cortisol response and mediated by a cortisol-induced increase in blood-to-brain glucose transport. 3. In response to decreasing plasma glucose levels the glycemic threshold for glucagon secretion normally liesjust below the physiological postabsorptive plasma glucose concentration range, and a decrease in p-cell insulin secretion is an important intraislet signal to a-cell glucagon secretion during mild to moderate hypoglycemia. 4. Hypoglycemia per se (insulin constant) stimulates the sympathetic nervous system (forearm norepinephrine spillover) and adrenergic symptoms (and cholinergic symptoms to the extent those are mediated by activation of cholinergic sympathetic neurons) of hypoglycemia are the result of the perception of physiological changes mediated by sympathetic neural as well as adrenomedullary activation. Testing of these hypotheses will address the mechanism (Aim 1)and mediator (Aim 2) of HAAF and the fundamental physiology of the key components of the pathophysiology of HAAF, loss of the glucagon (Aim 3) and neurogenic symptom (Aim 4) responses to falling plasma glucose concentrations. Insight into these basic issues can be expected to lead ultimately to clinical strategies that will minimize the risk of iatrogenic hypoglycemia and thus improve the lives of people affected by diabetes in both the short-term (improved quality of life) and the long-term (fewer complications).